Recent technology and basic photobiology are used to define, quantify, predict, and modify the optical properties and phototoxic reactions of skin and blood in a manner conducive to the exploration of ways to minimize risks and maximize the benefits of all forms of phototherapy and photochemotherapy. This is accomplished by applying sound methodology and dedicated electrooptical equipment to perform reliable optical measurements of skin and blood in vitro and in vivo, and utilizing this data to mathematically model cutaneous and cellular optical properties. Transmission of nonionizing electromagnetic radiation to various internal organs in the human body is also measured. Chemical agents which alter the optical properties of skin, increasing the penetration of selective wavebands of nonionizing electromagnetic radiation will be examined to determine the mechanism by which these alterations occur. Using specific scattering properties of skin, it is possible to utilize noninvasive optical means to quantify content of melanin and content, location and flow of blood in skin and thereby precisely measure degrees and depth profiles of the erythematous compgnent (vasodilatation) of skin inflammation. New exposure sources and the capability to quantify erythema permit extending measurement of the human erythema action spectra and dose-response curves to wavelengths longer than 315 nm with and without photosensitizers, and to wavelengths shorter than 250 nm. This quantified phototoxic endpoint also permits the first precise studies of waveband interactions, and the effects of temperature and infrared on photobiological responses of the intact skin of living man. Ultraviolet lasers will be used to achieve intensities not previously possible to study the limits of reciprocity (dose equals I x t equals photobiologic response) with and without photosensitizers. The lymphocyte will be used as an in vitro model to compare with and contrast with the in vivo cutaneous dose and response parameters of phototoxicity quantified by the proposed work.